Woodpulp is manufactured in the pulp and paper industry by various methods. Pulp can be manufactured chemically by cooking or mechanically by grinding and refining. It is also manufactured from waste paper by defibering the waste paper in a pulper. It is common to all pulp manufacturing methods that the pulp contains more or less impurities which must be removed therefrom. Pulp is cleaned in a screening plant by means of washers, thickeners, screens and cleaners. A screen is an apparatus in which pulp in the consistency range of about 1 to about 5% is cleaned by either a slotted screen or a perforated screen. The cleaner, normally a so-called centricleaner, is an apparatus in which pulp is cleaned by centrifugal force in a low, usually less than 1%, consistency. Thickeners and washers are used to remove from the fiber suspension liquor and impurities dissolved in the liquor, such impurities being in the form of small particles capable of passing through the filtering media. However, screening involves two major problems. First, it is usually desirable after screening to increase the pulp consistency to a range of about 10 to about 15% for storing or after-treatment. Secondly, handling of the reject is also desirable by either refining or some other method, but usually at a higher consistency than that present during screening. In other words, the pulp flows must always be thickened after screening. There have been attempts to resolve this problem such as, for example, by the Swedish company Kamyr AB. Their solutions aim at raising the consistency to 8-15% in the screening equipment. Efforts have been made in developing both screens and cleaners which will operate at a consistency of about 10%. However, this has been only partially successful. Screening and cleaning as such can be performed rather successfully at a high consistency, but the separation efficiency of the screens and cleaners is substantially decreased as the consistency increases. It can thus be said that Kamyr has replaced one problem with another, i.e. they have eliminated the need for thickening at the cost of decreased cleaning efficiency.